Well logging tools are well-known in the oilfield services industry. For example, propagation resistivity tools are used for resistivity logging downhole. It is well known that propagation resistivity tools require an offset calibration, usually called ‘air calibration’. The offset phase shift and attenuation depend on details of the tool construction and tool-to-tool variation. Those details may include, for example, machine tolerances in antenna grooves, shields types, variations in antenna and antenna shield positions, and collar diameters.
A typical method for determining the air calibration of a standard propagation resistivity tool is to suspend the tool in air far from conductive material such that the measured phase shift and attenuation is affected only by the tool body and is not affected by the environment. The results of this calibration are subtracted from the phase shift and attenuation log measurements before they are transformed into resistivities.
Retrievable propagation resistivity tools are made of two parts, namely a signaling portion having transmitter and receiver antennas constructed on a mandrel, and a tubular, such as a drill collar, into which the mandrel is inserted. For ease of discussion, “drill collar” will be used to mean the drill collar, tubular, or housing into which the mandrel may be removeably disposed. In any measurement sequence it is critical to know the “errors” that are caused by the signaling portion of the tool as well as those introduced by the structure around the signaling portion (e.g., the drill collar). Compounding the problem for retrievable tools, unlike standard tools, it is often desired to use individual signaling portions with different drill collars.
One method for calibrating signaling portions and drill collars is to perform calibration tests on various combinations of signaling portions and drill collars matched together as pairs. This is time consuming as well as costly. In actual practice such pairing is difficult to manage as signaling portions and drill collars can be moved from location to location independently. Thus, it is important to have a method to allow any signaling portion to be used with any drill collar without performing pre-calculations on that particular signaling portion/drill collar combination.